This invention relates to the need for alleviating the growing environmental problem of excessive plastic waste that makes up an ever more important volume fraction of what get thrown out in landfills every year. Biodegradable polymers and products formed from biodegradable polymers are becoming increasingly important in view of the desire to reduce the volume of solid waste materials generated by consumers each year. The invention further relates to the need for developing new plastic materials that can be used in applications where biodegradability, compostability or biocompatibility, are among primary desirable features of such applications. Such examples include for instance agricultural films, and the convenience that such films offer to farmers when they do not have to be collected after they have served their purpose. Flower pots or seeding templates are other examples where the temporary nature of the substrate translates into convenience for the user. Similarly, means of disposal of sanitary garments, such as facial wipes, sanitary napkins, pantiliners, or even diapers, may also be advantageously broadened with the use of materials that degrade in the sewage. Such items could be easily disposed directly in the sewage, after use, without disrupting current infrastructure (septic tanks or public sewage), and giving the consumer more disposal options. Current plastics typically used in making such sanitary garments cannot be disposed without undesirable material accumulation. New materials to be used in the examples above would ideally need to exhibit many of the physical characteristics of conventional polyolefins; they must be water impermeable, tough, strong, yet soft, flexible, rattle-free, possibly low-cost and must be capable of being produced on standard polymer processing equipment in order to be affordable.
Another application, which illustrates the direct benefit of compostable thermoplastic materials, is leaf/lawn bags. Today's sole compostable bag, which does not require the composter the additional burden of bag removal and the risk of compost contamination, is the paper bag. Yet, it fails to provide the flexibility, the toughness and moisture-resistance of plastic films, and is more voluminous to store. Compostable plastic films used to make leaf/lawn bags would provide bags that could be disposed much like paper bags, yet provide the convenience of plastic bags.
It becomes clear in view of these examples that a combination of biodegradability, melt-processability and end-use performance is of particular interest to the development of a new class of polymers. Melt processability is key in allowing the material to be converted in films, coatings, nonwovens or molded objects by conventional processing methods. These methods include cast film and blown film extrusion of single layer structures, cast or blown film co-extrusion of multi-layer structures. Other suitable film processing methods include extrusion coating of one material on one or both sides of a compostable substrate such as another film, a non-woven fabric or a paper web. Other processing methods include traditional means of making fibers or nonwovens (melt blown, spun bounded, flash spinning), and injection or blow molding of bottles or pots. Polymer properties are essential not only in ensuring optimal product performance (flexibility, strength, ductility, toughness, thermal softening point and moisture resistance) during end-use, but also in the actual product-making stages to ensure continuous operations. Rapid crystallization of the processed polymer melt upon cooling is clearly an essential feature necessary for the success of many converting operations, not only for economical reasons but also for the purpose of building in adequate structural integrity in the processed web (fiber, film) during converting, where for example crystallization times are typically less than about 3 seconds on commercial film and fiber lines.
In the past, the biodegradable and physical properties of a variety of PHA's have been studied, and reported. Polyhydroxyalkanoates are generally semicrystalline, thermoplastic polyester compounds that can either be produced by synthetic methods or by a variety of microorganisms, such as bacteria and algae. The latter typically produce optically pure materials. Traditionally known bacterial PHA's include isotactic Poly(3-hydroxybutyrate), or i-PHB, the high-melting, highly crystalline, very fragile/brittle, homopolymer of hydroxybutyric acid, and Poly(3-hydroxybutyrate-co-valerate), or i-PHBV, the somewhat lower crystallinity and lower melting copolymer that nonetheless suffers the same drawbacks of high crystallinity and fragility/brittleness. PHBV copolymers are described in the Holmes et al U.S. Pat. Nos. 4,393,167 and 4,880,59, and until recently were commercially available from Imperial Chemical Industries under the trade name BIOPOL. Their ability to biodegrade readily in the presence of microorganisms has been demonstrated in numerous instances. These two types of PHA's however are known to be fragile polymers which tend to exhibit brittle fracture and/or tear easily under mechanical constraint. Their processability is also quite problematic, since their high melting point requires processing temperatures that contribute to their extensive thermal degradation while in the melt. Finally, their rate of crystallization is noticeably slower than traditional commercial polymers, making their processing either impossible or cost-prohibitive on existing converting equipment.
Other known PHA's are the so-called long side-chain PHA's, or isotactic PHO's (poly(hydroxyoctanoates)). These, unlike i-PHB or PHBV, are virtually amorphous owing to the recurring pentyl and higher alkyl side-chains that are regularly spaced along the backbone. When present, their crystalline fraction however has a very low melting point as well as an extremely slow crystallization rate, two major drawbacks that seriously limit their potential as useful thermoplastics for the type of applications mentioned in the field of the invention.
Recently, new poly(3-hydroxyalkanoate) copolymer compositions have been disclosed by Kaneka (U.S. Pat. No. 5,292,860), Showa Denko (EP 440165A2, EP 466050A1), Mitsubishi (U.S. Pat. No. 4,876,331) and Procter & Gamble (U.S. Pat. Nos. 5,498,692; 5,536,564; 5,602,227; 5,685,756). All describe various approaches of tailoring the crystallinity and melting point of PHA's to any desirable lower value than in the high-crystallinity i-PHB or PHBV by randomly incorporating controlled amounts of “defects” along the backbone that partially impede the crystallization process. Such “defects” are either, or a combination of, branches of different types (3-hydroxyhexanoate and higher) and shorter (3HP, 3-hydroxypropionate) or longer (4HB, 4-hydroxybutyrate) linear aliphatic flexible spacers. The results are semicrystalline copolymer structures that can be tailored to melt in the typical use range between 80° C. and 150° C. and that are less susceptible to thermal degradation during processing. In addition, the biodegradation rate of these new copolymers is typically accrued as a result of their lower crystallinity and the greater susceptibility to microorganisms. Yet, whereas the mechanical properties and melt handling conditions of such copolymers are generally improved over that of i-PHB or PHBV, their rate of crystallization is characteristically slow, often slower than i-PHB and PHBV, as a result of the random incorporation of non-crystallizable defects along the chains. Thus, it remains a considerable challenge to convert these copolymers into various forms by conventional melt methods, for they lack sufficient structural integrity or they remain substantially tacky, or both, after they are cooled down from the melt, and remain as such until sufficient crystallization sets in. Residual tack typically leads to material sticking to itself or to processing equipment, or both, and thereby can restrict the speed at which a polymeric product is produced or prevent the product from being collected in a form of suitable quality. Hence, significant improvements in the rate of crystallization are needed if these more desirable copolymers are to be converted into films, sheets, fibers, foams, molded articles, nonwoven fabrics and the like, under cost-effective conditions.
The issue of the slow crystallization rate of PHBV is a well-recognized one and has been addressed previously either in the open literature or in patent applications which disclose a variety of options that can help enhance its crystallization rate. For example, Herring et al.'s U.S. Pat. No. 5,061,743 discloses the use of a combination of an organophosphonic acid or ester compound and a metal oxide, hydroxide or carboxylate salt as nucleating agents to improve the crystallization rates of PHA's such as PHB. It builds upon an earlier British composition patent by Binsbergen for crystalline linear polyesters (GB 1,139,528). Similarly, Organ et al. in U.S. Pat. No. 5,281,649 discloses the use of ammonium chloride as a nucleating agent to improve the crystallization rates of PHAs, for example PHB. The small size of the nucleant minimizes problems of opacity and agglomeration otherwise experienced with particulates. Additional examples of additives blended with PHA's that improve their crystallization rate can be found. For example, U.S. Pat. No. 5,516,565, to Matsumoto, proposes the use of crystallization agents such as aromatic aminoacids, e.g. tyrosine and phenyl alanine, that are capable of being decomposed or metabolized in an animal or in the environment, hence allowing the use of nucleated PHA in medical devices. In 1984, P. J. Barham wrote a review of the different types of nucleants in an article entitled “Nucleation behavior of poly-3-hydroxybutyrate” (J. Mater. Sci., 19, p. 3826 (1984)). He notes that the nucleating effect of impurities such as talc comes from their ability to reduce the entropy of partially adsorbed molecules, whereas additives such as saccharin work by epitaxial, crystallographic matching. He also described self-seeding, a phenomenon that produces an increase in the nucleation density of semicrystalline polymers, with however very limited practical implications since the polymer must be kept within only a few degrees of the peak melting point of the polymer. In a different article, Organ et al. also elucidate the epitaxial growth of PHB off ammonium chloride crystals and demonstrated positive results with boron nitride, saccharin and the hydrogen-peroxide salt of urea as nucleating agents (J. Mater. Sci., 27, p. 3239 (1992)). Finally, Hobbs et al. report about the beneficial effect of water on the crystal growth rate of thin films of poly(hydroxybutyrate) in a published article (Polymer, 38, p. 3879 (1997)).
Blends containing PHA's are also disclosed with potential benefits on their crystallization rate, and several scientific studies have been aimed at characterizing such blends. For instance, a Japanese patent assigned to Mitsubishi Rayon (JP Patent No. 63172762) reports on the use of i-PHB as an additive to PET in order to improve its crystallization rate. Kleinke et al., in U.S. Pat. No. 5,231,148, teach about a mixture containing polyhydroxyalkanoate and compounds with reactive acid and alcohol groups which possesses better mechanical properties and crystallizes at a higher temperature than the pure PHA. Hammond discloses polymer compositions containing a PHA polymer and an oligomer selected from the group: PHA's, polylactide, polycaprolactone and copolymers thereof (U.S. Pat. No. 5,550,173). In World Patent Application No. 96/09402, Cox et al. describe a hydroxycarboxylic acid copolyester comprising non-random blocks of different compositions, the higher melting component contributing to reduce the crystallization time of the overall material. In their scientific article published in Polymer, 34, p. 459 (1993)), Organ et al. examine the phase behavior and the crystallization kinetics of melt blends of i-PHB with PHBV (w/18.4% valerate) over their entire composition range, in 10% composition change increments. Their data indicate separate melt and two crystal phases in the case of blends that contain a majority of the PHBV copolymer. The authors however fail to recognize and establish positive consequences that such blend structures may have on their crystallization rate. In a scientific study published in Makrom. Chem., Makrom. Symp., 19, p. 235 (1988), Marchessault et al. describe the process of solution-blending i-PHB with PHBV in chloroform, followed by their co-precipitation in diethyl ether. Horowitz et al. describe an in-vitro procedure for preparing artificial granules made of i-PHB with PHO (using ultrasonic centrifugation) which produces a single, uniform population of granules that retain their amorphous elastomeric state (Polymer, 35, p. 5079 (1994)).
More immediately relevant to the present invention, Liggat in U.S. Pat. No. 5,693,389 discloses dry blending a higher melting PHA such as PHB in powder form to serve as a nucleating agent for a lower melting PHA such as PHBV. Although the idea has a positive impact on the crystallization rate, the crystallization rate benefit is limited by the relatively large size and the low dispersibility of the PHB powder. In addition, the size of the dispersed PHB powder generally impedes processing of such blends into thin products like films, coatings or fibers (due to die clogging), and can also be responsible for their low aesthetics and weakened mechanical properties (e.g.,stress concentration loci in the final articles, opacity, etc.). Moreover, the close vicinity of the i-PHB and PHBV melting points is responsible for the limited size of the processing temperature window where the nucleating i-PHB particles remain active. Very recently, Withey and Hay reinvestigated seeding phenonema and their influence on the crystallization rate in blends of i-PHB and PHBV (Polymer, 40, p. 5147 (1999)). Their approach however failed to generate better results for the use of i-PHB as a nucleating agent over boron nitride.
Hence, all prior reported attempts to improve the crystallization rates of PHA polymers and copolymers have been unsatisfactory in that the crystallization rate remains too low for commercial processing, and the nucleating agent can disadvantageously affect one or more properties of the polymer or copolymer, for example rendering them opaque or introducing loci of stress concentration, hence compromising the physical and mechanical or biodegradable properties of the polymers.
In addition to the above methods of chemical modification or blending of PHA's, there are also prior accounts of thermal treatment and special handling of PHA's that are said to contribute to increasing their crystallization rate as well as improving their physical properties. For instance, in U.S. Pat. No. 4,537,738, Holmes describes a process of preforming a partially crystallized PHB extruded form before subjecting it to a drawing stage and allowing completion of the crystallization in the stretched state. Waddington, in U.S. Pat. No. 5,578,382 proposes to achieve a high density of nucleation sites by cooling down a PHA film just above Tg (4-20° C.), before bringing the temperature back up towards the optimum temperature for crystal growth, for the purpose of achieving more rapid crystallization, smaller spherulites and improved barrier properties. De Koning et al. (Polymer, 34, p. 4089 (1993) & Polymer, 35, p.4599 (1994)) as well as Biddlestone et al. (Polym. Int., 39, p. 221 (1996)) studied the phenomena of physical aging and embrittlement in i-PHB or PHBV and attributed it to the occurrence of secondary crystallization with time. The phenomenon may be partially prevented or reversed by thermal annealing, by virtue of a change in morphology and a reduction of the overall amorphous-crystalline interface. De Koning (WO 94/17121) and Liggat et al. (WO 94/28047 and WO 94/28049) suggest the use of a post-conversion heating treatment to at least partially restore the mechanical properties of i-PHB or PHBV that are affected by physical aging and which is responsible for the embrittlement of the material over time. The same approach is proposed by Liggat et al (WO 94/28048) for these materials in the presence of a plasticizer.
Most of these process conditions applied to i-PHB or PHBV however fail to impart satisfactory physical and mechanical properties to the materials which generally tend to remain fragile. Accordingly, it would be advantageous to obtain PHA's which not only have improved crystallization rates, but also exhibit an advantageous combination of physical/mechanical properties allowing formation and use of shaped articles that are useful in a wide range of applications.